Closure control mechanism



July 18, 1961 H, BERKovn-z ETAL 2,992,818

cLosURE CONTROL MECHANISM Filed April 24, 1959 Harry Berkoviz 8 ATTORNEY 2,992,818 CLQSURE CONTROL WCHANISM Harry Berkovitz, Glen Rock, and Lennius Rissler, Hohokus, NJ., assignors to Westinghouse Electric Corporation, `East Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 24, 1959, Ser. No. 808,818 9 Claims. (Cl. 268-49) 'This invention relates to closure control mechanism and it has particular relation to controls for the doors of elevator systems.

Although aspects of the invention are applicable to closures designed for various applications, the invention is particularly suitable for closures or doors employed in elevator systems. Consequently, the invention will be described with particular relation to elevator closures or doors. Furthermore, aspects of the invention are applicable to doors of various types, such as center-opening or side-opening, attendant-operated or automatically-operated door assemblies. For the purpose of discussion, however, reference will be made to door assemblies of the center-opening type as employed in elevator systems.

An elevator car generally is provided with an entranceway so that load may be received into and discharged from the car at the various oors of the structure served by the elevator system with which the car is associated. The entranceway commonly is furnished with a door for opening and closing the entranceway at each floor.

Elevator doors may be manuallyor power-operated. In doors of the power-operated type, a door operator may be provided which includes motive means in the form of an electrically energized motor. Conveniently, a door operator for an elevator car door may be mounted on the elecator car. In addition, suitable control circuits may be provided for the motor -for controlling opening and closing movements of the door.

When closing or opening a relatively massive door, such as is employed for an elevator car, it is important that the door terminal velocities and forces be kept to a minimum. If they are not minimized, objectionable slamming and excessive vibration will occur as the door arrives at a terminal position. In order that the door may arrive at a terminal position with minimum force and velocity to provide quiet operation and to move the door between terminal positions as quickly as practicable, a door moving operation may begin by acceleration of the door away from its initial terminal position. As the door moves toward its opposite terminal position, it may be decelerated or checked in one or more steps, so that by the time the door arrives at its opposite terminal position its force and velocity are at a minimum.A

It will be appreciated, however, that under normal service conditions there are marked variations in ambient temperatures which effect changes in door operating motor speeds and in mechanical frictions. Furthermore, the door sill and supporting hanger conditions at and adjacent each terminal position of the door may be Widely different. Heretofore, in order to minimize `service calls, it was necessary to provide considerable leeway in the linal door closing and opening forces. A door may have been adjusted for satisfactory operation one day, but the following day it may have been either slamming or stalling as a result of the aforementioned variations.

In accordance with the invention, an elevator door of the type described is provided with an anti-stall feature. In a preferred embodiment of the invention, door accelerating force is applied subsequent to the application of door decelerating force in the event that transient friction tends to stall the door following the application of the decelerating force. Timing means may be provided which elfects the application of the accelerating Vforce provided rates Patent ICC the door fails to arrive at a terminal position within a predetermined time after the application of the decelerating force. Conveniently, the anti-stall feature may be applied to the door for both opening and closing movements thereof. An elevator door provided with the antistall feature disclosed herein may be adjusted so that under normal operating conditions, that is, in the absence of abnormal frictional stalling forces, the door will arrive at a terminal position with minimum velocity and force to insure quiet operation.

It is, therefore, an object of the invention improved closure control mechanism.

It is another object of the invention to provide a poweroperated door operating mechanism with anti-stalling means.

It is a further object of the invention to provide an elevator car having a power operated door and decelerating means for the door as it approaches a terminal position, wherein accelerating force is applied to the door in the event that the door tends to stall after operation of the decelerating means.

It is an additional object of the invention to provide an elevator door with anti-stalling means which become operative if the door `fails to reach a terminal position during a door moving operation within a predetermined time after its arrival `at a predetermined position during its approach toward the terminal position.

Other objects of the `invention will be apparent from the following description, taken in conjunction with the accompanying drawings, in which:

FIGURE l is a view in front elevation, with portions broken away and parts not shown, of an elevator car having a closure System embodying the invention; and

FIG. 2 is a schematic view of an electrical control system for operating the closure of FIG. l.

Referring to the drawings, FIG. l illustrates an elevator car having a door 1 for opening and closing an elevator car entranceway 3 through which load may enter and leave the car. This elevator car may serve any desired number of lioors or landings. Since suitable control mechanisms for elevator cars are understood in the art, further discussion thereof is unnecessary for an understanding of the invention.

As heretofore noted, the elevator card door may be of any desired conventional construction, such as a centeropening or a side-opening door or a double or a single door. For purpose of illustration, it will be assumed that the door is a center-opening door mounted for horizontal sliding movement.

A door operator mounted on the elevator car -is employed for opening and closing the car door to expose and to close the entranceway 3. Such an operator is shown in FIG. l and now will be described.

The `center-opening car door l1 comprises two sections 5 and A5. In FIG. l the door is shown in its fully closed position. A number of similar components are employed for the door sections S and A5. Insofar as is practicable, a component for the door section A5 which is similar to a component for the door section 5 will be identified by the same reference numeral as is employed `for the corresponding component associated with the door section 5 prefixed by the letter A.

The door section 5 is provided with a door hanger 7 on wlhich door hanger wheels 9 are mounted for rotation. The door hanger Wheels for the door sections 5 and AS are positioned for movement along a horizontally-mounted track 11 in a conventional manner. The track 11 is secured to the elevator car by any suitable means.

Movement of the door section 5 is effected by a lever 1-3 pivotally mounted on the elevator car by means of a pin 15. The lower end of the lever 13 is pivota'lly to provide connected to one end of a link 17, the other end of the link being pivotally connected to the door section 5. By inspection of FIG. l, it will be observed that the lever 13 is coupled to the lever A13 by a link 19, the ends of which are pivotally attached to the levers 13 and A13 by pivots 21 and A21, respectively. It will be observed that the pivot 21 is positioned above the pin 15, whereas the pivot A21 is located below the pin A15. Consequently, rotation of the lever 13 to open the door section moves the link '19 in the proper direction to open the door section A5.

The lever 13 preferably is operated by a suitable door operator 23 which may include a reversible electric motor 25 coupled through suitable gearing to a shaft 27. The shaft 27 carries an arm 29 which is pivotally connected to one end of a flink 31, the rem-aiming end of the link 31 being pivotally connected to the lever y13. Consequently, the motor 25 may be energized in a conventional manner for the purpose of opening and closing the door sections 5 and A5. When the door 1 is to be opened, the motor 25 is operated to rotate the arm 29 in a clockwise direction as viewed in FIG. l. In order to reclose the door, the electric motor is reversed and returns the arm 29 to the position illustrated in FIG. l.

A control assembly 33 is mounted on the elevator car adjacent the motor 25. Positive driven contact cams located in the control assembly 33 control the rate of acceleration and deceleration of the door 1 as will be described below. The control assembly also houses control contacts and control resistors. The contact cams are keyed to the gearing associated with the motor 25 and operate the control contacts for predetermined distances of travel of the arm 29 to vary motor armature circuit resistance, thus controlling the doors rate of acceleration and of deceleration. Each cam is symmetrical and operates two spring-closed contacts, one contact being located on each side of the cam. For each direction of door movement, a separate and identical set of contacts is actuated, one for the opening movement of the door, the other for the closing movement of the door. Such arrangement is Well known in the art.

In order to illustrate suitable operation of the door controller 33, a schematic control diagram is shown in FIG. 2. In this diagram, the armature 25A and the eld winding 25F of the door operating motor 2.5 (FIG. l) are illustrated. Electrical energy for the control circuits is derived from a pair of direct-current buses L+ and L It will be observed that the motor field Winding 25F is connected directly across the buses L-land L In parallel with the field winding 25F is a rectifier 35 of a conventional type, such as selenium. Current flows through the rectifier 35 in the direction indicated by its circuit symbol in FIG. 2. Thus the rectifier 35 provides a path for induced current as a result of the collapse of the motor field windings magnetic eld in the event that power is removed from the buses L-land L The motor 25 is energized to open or to close the car door by operation of a switch SW. Although this may be a manually operated switch, in a preferred embodiment of the invention this switch represents the contacts of a relay or relays employed in any conventional door operating system to initiate `an opening or a closing operation of the door. Thus, movement of the operating member of the switch SW up, as viewed in FIG. 2, to close its contacts SW1 completes, with a limit switch 37 and break contacts CL1 of a door closing relay CL, a circuit connecting a door opening relay OP across the buses L+ and L for energization. The limit switch 37 is opened as the door arrives at its fully open position by a cam located in the control assembly 33.

Movement of the operating member of the switch SW down results in closure of its contacts SW2 to cornplete, with a .limit switch 39 and break contacts OP1 of the door opening relay OP, a circuit connecting the door closing relay CL across the buses L-land L for ener- -gization The limit switch 39 is opened as the door arrives at its fully closed position by a cam located in the control assembly 33.

It will be observed that the break contacts CL1 prevents energization therethrough of the door opening relay OP when the door closing relay CL is energized. The break contacts OP1 operate in a similar manner in the circuit of lthe door closing relay CL. By inspection of FIG. 2, it will be seen that also associated with the relay OP are make contacts OP2 and OP4 and break contacts OPS. Associated with the relay CL are make contacts CL2 and `CL4 and break contacts CL3. 'I'hese contacts control energization of the motor armature 25A, the circuits for energization of the armature being located in the flower portion of FIG. 2.

Associated with the armature 25A are a plurality of adjustable resistors and a plurality of cam-operated control contacts for controlling acceleration and deceleration of the motor 25. These resistors and contacts, together with the contact cams for the latter, are located in the control assembly 33 as above described.

l't will be noted that the adjustable resistor 41 is disposed in series circuit relationship with the armature 25A in the bus L|-. The remainder of the adjustable resistors associated with the amature 25A bear identifying symbols which are indicative of their functions. Thus, the adjustable resistor RAC is employed to effect acceleration of the motor during a door closing operation while the adjustable resistor RAO is employed for accelerating the motor during door opening movement. Similarly, the resistor RDCI is used for decelerating the motor and thereby the door during a door closing movement while the adjustable resistor RDOI effects deceleration of the motor during door opening movement. Likewise, the cam operated control contacts bear identifying symbols which are indicative of their control functions. For example, the contacts AC and AO are effective for accelerating the door during door closing and door opening movements, respectively. The contacts DC1 through DS4 effect deceleration of the door during door closing movement and operate sequentially in the order of their suffix numerals. The contacts D01 through D04 in sequence similarly control door deceleration during a door opening operation.

It Will be observed that make contacts ASC and ASO are disposed in series circuit relationship with the camoperated contacts DC4 and D04, respectively. These contacts are associated with an antiestall or checkback relay AS. If a pair of limit switches 43 and 45 both are in closed condition, the relay AS is connected for energization across the buses L-I- and L In parallel with the coil of the relay AS is a resistor-capacitor network comprising serially connected resistors 47 and 49 and a capacitor 51. In parallel with the resistor 47 is a rectifier 53 of a conventional type such as selenium. Current flows through the rectifier 53 in the direction indicated by its circuit symbol in FIG. 2. Thus, When both of the limit switches 43 and 45 are in closed condition, the capacitor 51 charges through the resistor 49 and the rectifier 53, which, in effect, then shorts the resistor 47. When one of the limit switches 43 or 45 is opened, the capacitor 51 discharges through the resistors 47 and 49 and the coil of the anti-stall relay AS. Since the length of times of charge and discharge of the capacitor are dependent upon the RC network time constant, the rectifier 53 effects a fast charge of the capacitor 51 and a relatively slow discharge thereof.

The limit switches 43 and 45 are located in the control assembly 33 and are operated by cams disposed therein. In a preferred embodiment of the invention, the cam associated with the switch 43 operates to open the switch simultaneously with the opening of the control contacts DC4 by its associated Vcontact cam. The limit switch 45 ing door opening and door closing movements.

is opened by its cam simultaneously with the opening of the control contacts D04 by its associated contact cam. Each of these limit switches remains in open condition from the time of its opening to the time when the door reaches the same position in a door movement opposite in direction to that in which the door was moving when the respective limit switch was opened by its associated cam.

In order that the invention may be fully understood, typical door operations now will be considered. The conditions of various control contacts and limit switches in FIG. 2 are shown as they appear in actual operation when the door is in fully closed position, as is illustrated in FIG. 1. It tirst will be assumed that the operating member of the switch SW is moved up as viewed in FIG. 2. Closure of the switch contacts SW1 results in energization and pickup of the door opening relay OP, since the limit switch 37 is in closed condition. Pickup of the relay OP results in opening of its break contacts OPI and P3 and closure of its make contacts 0P2 and 0P4. Closure of the make contacts OP2, and 0P4 results in energization of the armature 25A of the motor 2S through the following circuit:

L+, 41, OP2, 25A, D01, RAOl, RAOZ, OP4, L

Opening of the break contacts 0P3 prevents the flow of :current through the adjustable resistor 55.

Energization of the armature 25A results in clockwise rotation of the shaft 27 (FIG. l) to initiate opening movement of the door sections and A5. The resistor 41 is adjusted to control the over-all speed of the motor dur- The adjustable resistor RAO has an arm RAOA which is adjusted to control the initial torque developed by the motor 25. As the door begins to open, the contact cam associated with the control contacts AO closes the contacts A0 to short out a portion RAO2 of the resistor RAO. Since the total resistance in series with the armature 25A now has decreased, armature current increases to accelerate the motor 25 and thereby the door sections 5- and A5. An arm RAOB of the resistor RAO is adjusted to control the magnitude of this acceleration. Y

As the door sections continue to move, the control contacts D01 are opened by their associated contact cam to insert a portion of the adjustable resistor RDOI in series with the armature 25A. Armature current thus decreases to decelerate or check fthe motorand thereby the door sections. The resistor RDOI is adjusted to obtain the desired amount of deceleration at this point.

Further movement of the door and operation of contact cams and their associated control contacts results in the sequential closing of the control contacts D02, D03 and D04 to short out portions RDOZ, RD03 and RD04, respectively, of the adjustable resistor 57. (As heretofore explained, the anti-stall relay AS is picked up and its make contacts ASO, therefore, are closed.) Since the resistor 57 is in parallel with the armature 25A, the sequential shorting out of various portions thereof results in the shunting of the armature by successively lower resistances. As the total resistance shunting the armature decreases, more current is drawn away from the armature through the shunt path, resulting in increased deceleration of the motor and thereby of the door sections 5 and A5. The arms of the resistor 57 are adjusted to obtain the desired magnitude of deceleration of the door sections at each deceleration or check point, as determined by operation of the control contacts D02, D03 and D04.

Conveniently, in a typical installation the last check point, las determined by closure of the control cont-acts D04, may be located approximately one and one-half inches from the fully open position of the door 1; and the arm of the resistor 57 `associated with the contacts D04 may be adjusted so that under normal conditions approximately one and one-half seconds are required for the door sections to tr-avel from this point to their fully open positions. It will be recalled that at this point the limit switch 45 is opened by its associated cam.. Although the door sections travel' relatively slowly from this point to their fully open positions, under normal conditions the torque of the motor 2S is sufficient to bring the door sections to their fully open positions before the capacitor 5'1 discharges suiciently through its associated resistors to drop out the anti-stall relay AS. lf, however, abnormal frictional force prevents the full opening of the door before the relay AS drops out, opening of the contacts ASO as a result of such dropout results in reinsertion of the portion RD04 of the adjustable resistor 57 in parallel with the armature 25A. Such increase in armature shunting resistance effects a corresponding increase in current owing through the armature 25A. As a result thereof, the motor 25 checks back to develop increased torque for applying accelerating force to the door sections 5 and A5 to overcome the abnormal frictional force, thereby accelerating movement of the door to its fully open position. Conveniently, the resistors 47 and 49 and the capacitor 51 may be selected to effect dropout of the anti-stall relay AS within -approximately two and one-half seconds after opening of the limi-t switch 45.

As the door arrives at its fully open position, the limit switch 37 is opened to drop out the door opening relay OP. Closure of the break contacts OPi prepares the door closing relay CL for subsequent energization through the switch contacts SW2.

Since the door closing control contacts and adjustable resistors associated with the armature 25A are in all respects symmetrical with the door opening control contacts and adjustable resistors associated therewith, it `appears unnecessary .to describe them in detail. Suffice it to say that the anti-stall relay AS and its associated m-ake contacts ASC operate in a manner similar to that described for its make contacts ASO. Thus, if during a door closing operation frictional stalling force is applied to the door subsequent to the final check point as determined by operation of the control contacts D04, the relay AS wil-l drop out to open its contacts ASC for effecting checkback of the armature control circuits to the next-to-last decelerating point. As `a result, accelerating force will be applied to the door to overcome the abnormal frictional stalling force.

It is obvious that the checkback contacts of the antistall relay AS could be shifted to any other point. For example, they could` be shifted to the second check (between lthe control contacts D02 and D03 and/ or between the contacts D02 and D03) if so desired. 0r it may be preferable in certain applications to position the checkback contacts ASO land/ or ASC in the circuit so that full line voltage is applied to the armature upon dropout of the anti-stall 4relay AS. In such instance, even greater force will be applied t-o the `door to overcome abnormal stalling force.

It will be appreciated that the invention disclosed herein provides means for maintaining minimum door terminal velocities and forces under normal conditions to effect quiet door operation by preventing objectionable slamming =and vibration as the door sections arrive at their terminal positions. At the same time, means are provided for overcoming abnormal stalling forces which may be applied to the door due to transient variations in ambient temperatures and in door sill and hanger conditions.

An additional advantage of the invention is that the size of the door operating motor may be rela-tively small, since, in order to obtain relatively large motor torque at relatively low motor speed for safe operation, the size of the motor must be increased. Both the weight and the cost of a door operating motor may be decreased Where it is permissible to use relatively low torque at relativelyl low motor speed. The use of a smaller motor results in the decrease of motor power requirements and motor contactor size. This results in decreased cost and space requirements. Additionally, lower motor power or current requirements will effect a decrease in the size of control Wiring.

It should be noted that there are methods well known in the art other than timing means .for determining when stalling `force is applied to the door and to the door operator. For example, velocity, torque or force responsive means may be employed. Alternatively, means responsive to the magnitude of motor armature current or voltage may be used. Furthermore, such stall-indicating means may be utilized to effect the instantaneous application of fanti-stalling force to the door rather than upon the expiration of a time delay as described above.

in modern elevator systems, a hoistway door generally is provided for the hoistway in which an elevator c-ar travels Iat each floor served by the oar. Opening and closing of such hoistway doors may be effected by a master care and hoistway door operator which is mounted on the elevator car, the hoistway door being controlled through cooperating vane `and drive block members which are well known in the art. Alternatively, each hoistway door may be controlled by a separate operator similar to that herein `described for the car door. Obviously, the invention may be applied both to master ydoor operators and to separate hoistway door operators.

While the invention has been described with reference to certain speciiic embodimen-ts thereof, numerous modications falling within the spirit and scope of the invention are possible.

We claim as our invention:

l. A closure system comprising an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force to move the door, and control means operable in cooperation with the motive means for moving the door with a predetermined force pattern between a fully open position and a fully closed position, said control means including means responsive to a predetermined system condition for'increasing said force to -a value above the value called for by said force pattern as the door is moved between its rfully open and fully closed positions by said motive means.

2. A closure system comprising an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force to move the door, and control means operable in cooperation with the motive means for moving the door with a predetermined force pattern between a fully open position and a fully closed position, said control means including modifying means responsive to abnormal delay in completion of a predetermined part of a door-moving operation between said positions by said motive means for modifying said force pattern to increase the force exerted by said motive means on said door to a value greater than the value called for by said force pattern, and means rendering said modifying means capable of initiating said modification only when the door is located intermediate said fully open and closed positions.

3. An elevator car having an entranceway, `a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force to move the door, and control means operable in cooperation with the motive means for moving the door between a fully open terminal position and a fully closed terminal position, said control means including check means for reducing the force exerted on the door by said motive means as it is moved toward one of said terminal positions by said motive means, and means for determining abnormal delay in completion of a predetermined part of a door-moving operation toward said one terminal position by said motive means, and means responsive to said determination by the last-named means for rendering said check means inoperative and for increasing the force exerted on the door by Said TQQ- tive means.

4. An elevator car having an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force on the door to move the door, and control means operable in cooperation with the motive means for moving the door between a fully open terminal position and a fully closed terminal position, said control means including force-reducing means responsive to the arrival of the door at a predetermined position intermediate said terminal positions during a door-moving operation by said motive means toward one of said terminal positions for decreasing the force exerted on the door by said motive means, timing means operable from `a rst condition to a second condition after a predetermined time delay, means responsive to larrival of the door at said predetermined position for initiating said time delay, said time delay being for a period of time which is longer than the time normally required for said motive means to move the door between said predetermined position and said one terminal position, and means responsive to the operation of said timing means to its second condition before the door arrives at said one terminal position because of abnormal delay in completion of movement of the door from said predetermined position to said one terminal position for rendering said force-reducing means inoperative and for effecting the application of increased force on the door by said motive means.

5. An elevator car having an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force on the door to move the door, and control means operable in cooperation with the motive means for moving the door between a fully open terminal position and a fully closed terminal position, said control means including first check means operative lat rst predetermined door position-for reducing the force exerted on the door by said motive means as it is moved toward one of said terminal positions by said motive means, second check means operative `at a second predetermined door position for further reducing the force exerted on the door by said motive means as it continues movement toward said one terminal position, timing means operable from a rst condition to a second condition after a predetermined time delay, means for initiating said time delay coincident with said reduction in force by said second check means, said time delay being for a period of time which is longer than the time normally required for said motive means to complete movementof the door from said second predetermined position to said one terminal position, and check shifting means responsve to the operation of said timing means to its second condition before the door arrives at said one terminal position because of abnormal delay in completion of movement of the door from said second predetermined position to said one terminal position for transferring said motive means ttrom control by said second check means to control by said rst check means, whereby increased force is applied to the door by said motive means.

6. An elevator car having an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force on the door to move the door, and control means operable in cooperation with the motive means for moving the door between a fully open terminal position and a fully closed terminal position, said control means including timing means operable from a rst condition to a second condition after a predetermined time delay, means responsive to arrival of the door at a predetermined position intermediate said terminal positions during a door moving operation toward one of said terminal positions by said motive means for initiating said time delay, said time delay being for a period of time which is longer than the time normally required for said motive means to complete movement of the door from said predetermined position to said one terminal position,

and means responsive to the operation of said timing means to its second condition before the door arrives at said one terminal position because of abnormal delay in completion of movement of the door from said predetermined position to said one terminal position for effecting the application of increased force to the door by said motive means.

7. A closure system comprising an entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force to move the door, and control means operable in cooperation with the motive means for moving the door with a predetermined force pattern between a fully open position and a fully closed position, said control means including -means responsive to abnormal delay in completion of a predetermined part of a dooropening operation by said motive means for increasing the -force exerted by said motive means on said door to a value greater than the value called for by said force pattern.

8. A closure system comprising an entranceway, a door yfor the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force on the door to move the door, and control means operable in cooperation with the motive means for moving the door with a predetermined force pattern between a fully open terminal position and a fully closed terminal position, said control means including timing means operable from a first condition to a second condition after a predetermined time delay, means responsive to arrival of the door at a predetermined position during a door moving operation toward one of said terminal positions by said motive means for initiating said time delay, said time delay being for a period of time which is longer than the time normally required for said motive means to complete movement of the door from said predetermined position to said one terminal position, and forcemodifying means responsive to the operation of said timing means to its second condition before the door arrives at said' one terminal position because of abnormal delay in completion of movement of the door from said predetermined position to said one terminal position for increasing the force exerted by said motive means on the door to a value greater than the value called for by said force pattern.

9. A closure system comprising a-n entranceway, a door for the entranceway, means mounting the door for movement to close and to open the entranceway, motive means for exerting force on the door to move the door, and control means operable in cooperation with the motive means for moving the door with a predetermined force pattern between a fully open position and a fully closed position, said control means including timing means operable from a first condition to a second condition after a predetermined time delay, means responsive to arrival of the door at a first predetermined position between said fully open and closed positions during a door moving operation by said motive means for initiating said time delay, and force-modifying means responsive to the operation of said timing means to its second condition before the door arrives at a second predetermined position during said door moving operation because of abnormal delay in completion of movement of the door from said first to said second predetermined I.position for increasing the force exerted by said motive means on the door to a value greater than the value called for by said force pattern, said time delay being for a period of time which is longer than the time normally required for said motive means to complete movement of the door from said first to said second predetermined position.

References Cited in the rile of this patent UNITED STATES PATENTS 

